Exchange transfusions of plasma, plasma fractions, leucocytes and plasma plus leucocytes from partially hepatectomized rats to non-hepatectomized animals will be used to initiate hepatic DNA synthesis. Initiating factors of hepatic regeneration of non-portal origin will be investigated by similar exchange transfusions from animals that have undergone removal of portal viscera as well as partial hepatectomy. Cultured adult hepatocytes will be used as an assay system for non-portal initiating factors of hepatic regeneration after demonstration of their activity in in vivo studies.